PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 813 



that this progression may be interrupted by indrafts of water from offshore, or that 

 the seasonal schedule may vary from year to year. 



The 100-meter salinities for this locality have averaged about 32.9 to 33 per mille 

 for the period February to July (extremes 33.8 and 32.5 per mille), with no definite 

 seasonal variation during that period. All but one of the determinations for the 

 period August to October have been appreciably lower (32.5 and 32.6 per mOle) than 

 any for the rest of the year, however. An average seasonal varition of about 0.3 

 per mille is thus indicated at 100 meters, reflecting the extreme depth to which vernal 

 freshening from above is effective; but here, near its lower limit, this freshening does 

 not culminate until a month or two later than at 40 meters, or four months later than 

 at the surface. 



The data collected so far faU to show whether any definite seasonal variation of 

 this sort can be traced at depths greater than 100 meters at this locality. 



Closer to land, in Massachusetts Bay off Boston Harbor, vernal freshening 



effects about as great a decrease in the salinity of the surface as at the mouth — 



from 32.1 to 32.2 per mille in March (of 1920 and 1921) to about 31 per mille in 



April and to about 30 per mille in May, followed by rather rapid recovery to 31 to 



32 per mille through July and August. The lowest values have been recorded as 



early in the year at 40 meters as at the surface (about 31.6 to 31.7 per mille, April 



and May, 1920). 



OFFING OF THE MERRIMAC RIVER 



The truly remarkable extent to which the vernal discharges from the large rivers 

 govern the seasonal cycle of salinity in the coastwise belt of the gulf is illustrated by 

 the ofRng of the Merrimac. To the southward of the Isles of Shoals, in its train, 

 vernal freshening is as sudden an event and the decrease in the salinity of the sur- 

 face is as great (by about 4 per mille) as in the Bay of Fundy (p. 808) ; but in 

 the trough between the Isles of Shoals and Jeffreys Ledge, only some 20 miles out 

 from the mouth of the river, the extreme range of salinity so far recorded at the 

 surface for the months of December, March, April, May, July, August, October, and 

 November" has been only about 1.2 per mille (31.6 to 32.8 per mUle) ; nor does ver- 

 nal freshening seem to culminate there until August — three months later than along 

 shore. Furthermore, its effect is so closely confined to the immediate surface here 

 that it has little effect at 40 meters and is not definitely reflected at all in the records 

 for 100 meters or deeper where the salinity has proved virtually constant from sea- 

 son to season and with but slight variations from year to year. 



NEAR MOUNT DESERT ISLAND 



The vernal freshening of the surface culminates at about the same season near 

 Mount Desert Island as in the Bay of Fundy — i. e. late in April or early in 

 May.'* However, this sector of the coast is so much less affected by river water, 

 and so much more open to the offshore waters of the gulf, that the seasonal range 



" A total of 10 stations. 



" Although only 12 sets of salinities have been taken here, the fact that we have records for 6 consecutive months for 1915, and 

 that the other data are consistent with these, makes the graph a reliable picture of the cycle for the half year. May to October, 

 which covers the season when the greatest changes in salinity take place. 



